27 Endoscopic Approach to the Fourth Ventricle
27.1 Introduction
The fourth ventricle was originally described by Galen1 and Da Vinci, and described as part of the ventricular system by Vesalius, but it remained a structure poorly studied until the nineteenth century. Magendie described the central foramen,2 and Luschka the lateral foramina.3 A surgical approach to it was not possible until the early twentieth century, when it was studied by Cushing4 and Dandy. The implementation of microsurgical techniques allowed neurosurgeons to reach the fourth ventricle in a safe and feasible way. The development of the flexible neuroendoscopy started a new era for intraventricular endoscopy.5 Nevertheless, an endoscopic approach to the fourth ventricle remains challenging.6 For the majority of neurosurgeons, it has been difficult to believe that the fourth ventricle could be reached and explored through a transventricular and transaqueductal route, because of the potential risks. To do so is now a reality, as Longatti and Torres-Corzo have demonstrated.7,8,9
Currently there are well-defined flexible neuroendoscopic procedures to manage ventricular and subarachnoid space pathologies. These procedures have been classified into basic and advanced.8,9,10 The exploration of the fourth ventricle is considered an advanced endoscopic procedure. In general, pathologies within the fourth ventricle are associated with hydrocephalus; therefore, an endoscopic approach serves two goals: diagnosis and possible endoscopic treatment of hydrocephalus. In this chapter, we will review in detail the indications, operative technique, clinical results, and potential complications of the endoscopic approach to the fourth ventricle.
27.2 Indications/Contraindications
In our experience, an endoscopic approach with a flexible neuroendoscope to the fourth ventricle is indicated in tri- or tetra-ventricular hydrocephalus of unknown etiology, trapped or isolated fourth ventricle, neurocysticercosis of the fourth ventricle for parasitic extraction and reopening of medial and lateral foramina (see Chapter 28), hydrocephalus secondary to congenital obstruction of the fourth ventricular outlets, hydrocephalus secondary to inflammatory and infectious ventriculitis and/or arachnoiditis, hydrocephalus secondary to fourth ventricle hemorrhage, and hydrocephalus associated with tumors of the fourth ventricle.10,11,12,13,14
In general, exploration of the fourth ventricle is focused on restoring cerebrospinal fluid flow into the cisterna magna. This procedure can be performed on patients of any age, including newborns.15
Relative contraindications to the procedure are acquired or congenital stenosis of the aqueduct, very small fourth ventricle, and inexperience with the procedure. With patience, a good and careful endoscopic technique can be developed, and the neuroendoscopist can routinely perform this procedure.
27.3 Operative Technique
27.3.1 Preoperative Preparation
Under general anesthesia, the patient is placed supine and the head slightly flexed, at 30º. As for any endoscopic procedure using the flexible neuroendoscope, the ideal site for the bur hole is 2.5 cm to the right of the midline and discreetly in front of the coronal suture.
27.3.2 Description of the Procedure
Once the bur hole is made, a cruciate durotomy and corticotomomy are performed, and the ventricle is punctured with a ventriculostomy needle aimed at the foramen of Monro. Samples of the ventricular fluid are taken. The ventricular needle is replaced with a 8-Fr peel-away sheath and the flexible endoscope is introduced into the right lateral ventricle.
The endoscope is held fixed using an arm holder. We advocate the use of the four-hands technique, in which one neuroendoscopist has control of the endoscope, advancing, retreating, and rotating the tip, while the second neuroendoscopist controls irrigation, instrumentation, and the steerable tip nub. Both neuroendoscopists must be in constant coordination.
Once in the lateral ventricle, inspection of the ventricle is performed. The endoscope is inserted into the third ventricle, and the anterior and posterior portions are carefully inspected. The endoscope is passed under the interthalamic mass, and the cerebral aqueduct entrance (aditus aquaeducti) is identified. Once identified, the endoscope is gently introduced into it (Fig. 27.2a), and while the first neuroendoscopist rotates the endoscope 45º along its axis counterclockwise, the second neuroendoscopist adjusts the tip of the endoscope with a slight medial bending to address the endoscope into the second portion of the aqueduct. At this point, the first neuroendoscopist makes another 45º rotation counterclockwise, and the second endoscopist sets the tip correcting the medial flexion to go through the third and fourth portions of the aqueduct and reach the upper portion of the fourth ventricle. At this point the endoscopic view will be reversed (180º), the upper part of the surgical field will become the lower part of the view. The left-right relationship of surgical field to endoscopic view remains unchanged. Operators must be fully aware of this to avoid accidents. Once the endoscope is in the fourth ventricle, a nub movement is performed to test the orientation and location of the endoscope’s steerable tip within the fourth ventricle (Fig. 27.1a,b).
Endoscopic Analysis of the Fourth Ventricle
Overview
While we keep the endoscope in neutral position in the uppermost portion of the fourth ventricle, we will observe the following structures: the cerebellar vermis at 12 o’clock, the floor of the ventricle with the striae medullares and foramen of Magendie in the back at 6 o’clock, the right Luschka foramen at 3 o’clock and the left at 9 o’clock. Finally the choroid plexus is seen toward center to the medial and lateral foramina16,17 (Fig. 27.2b).
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